The chambered nautilus (Nautilus pompilius), also called the pearly nautilus, is the best-known species of nautilus. The shell, when cut away, reveals a lining of lustrous nacre and displays a nearly perfect equiangular spiral, although it is not a golden spiral. The shell exhibits countershading, being light on the bottom and dark on top. This is to help avoid predators, because when seen from above, it blends in with the darkness of the sea, and when seen from below, it blends in with the light coming from above.
The range of the chambered nautilus encompasses much of the south Pacific; It has been found near reefs and on the seafloor off the coasts of Australia, Japan, and Micronesia.
The oldest fossils of the species are known from Early Pleistocene sediments deposited off the coast of Luzon in the Philippines.
thumb|The first and so far oldest known fossil of [[Chambered Nautilus|chambered nautilus, displayed at the Philippine National Museum]]
Although once thought to be a living fossil, the chambered nautilus is now considered taxonomically very different from ancient ammonites, and the recent fossil record surrounding the species shows more genetic diversity among nautiluses now than has been found since the extinction of the dinosaurs. Indeed, the taxon of the chambered nautilus, Nautilus pompilius, is actually a grouping of tens of different species of nautilus under one name. In 2016, they were moved to CITES Appendix II, which restricts international trade, and later the chambered nautilus was recognized as a threatened species under the Endangered Species Act.
Life cycle
Because of their oceanic habitat, studies of their life cycle have primarily been based on captive animals and their eggs have never been seen in the wild. Although nautilus have been kept at public aquariums since the 1950s, the chambered nautilus was first bred in captivity at the Waikiki Aquarium in 1995 (a couple of other nautilus species had been bred earlier) and captive breeding remains a rare event even today. Unlike most cephalopods, the nautilus lacks a larval stage. The eggs are laid in crevices or between corals by the female. The nautilus shell of the young develops inside the egg and breaches the top of the egg before the nautilus fully emerges. Depending on water temperature, the eggs hatch after between 9 and 15 months. Like other nautilus but unlike most other cephalopods, chambered nautilus are relatively long-lived and only reach maturity when about 5 years old. Mainly scavengers, chambered nautiluses have been described as eating "anything that smells". have been recorded from Indonesia and northern Australia. This giant form was described as Nautilus repertus, but most scientists do not consider it a separate species.
N. p. suluensis is a much smaller animal, restricted to the Sulu Sea in the southwestern Philippines, after which it is named. The largest known specimen measured 160 mm in shell diameter.
Shell geometry
thumb|upright|1896 skiagraph ([[X-ray photograph) of a N. pompilius by James Green & James H. Gardiner, from Proceedings of the Malacological Society of London, Vol. II, Pl. XV, p. 178]]
The chambered nautilus is often used as an example of the golden spiral. While nautiluses show logarithmic spirals, their ratios range from about 1.24 to 1.43, with an average ratio of about 1.33 to 1. The golden spiral's ratio is 1.618. This is visible when the cut nautilus is inspected.
Shell function
The shell of the chambered nautilus fulfills the function of buoyancy, which allows the nautilus to dive or ascend at will, by controlling the density and volume of the liquid within its shell chambers. This was found during research in New Caledonia on nautiluses whose shell chamber fluid densities were tested at various depths, weeks apart.